1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the same.
2. Description of the Prior Art
An impurity doped polysilicon layer is commonly used in a MOS type semiconductor device as its gate electrodes and interconnection layers. A so-called polycide construction consisting of a layer of metal or metal silicide over a layer of polysilicon is sometimes incorporated to reduce the resistivity of polysilicon.
FIG. 1 is a cross-sectional view showing the vertical cross section of a MOS type semiconductor device having such a polycide construction. In the figure, a gate electrode 7 is formed on a gate oxide 3 in the central active region surrounded by a thick field oxide 2 for isolation, the gate electrode 7 being formed of a multilayer consisting of a polysilicon layer 4 with doped impurities and a metal silicide layer 5. In the semiconductor substrate 1 at the periphery of the gate electrode 7, source and drain regions 8, i.e., diffusion regions with impurities highly doped through a gate oxide layer 3, are formed.
The metal silicide 5 of the gate electrode 7 is a silicide of metal having a high melting point refractory and is, for example, titanium silicide, molybdenum silicide and so on. The metal silicide 5, together with the impurity doped polysilicon layer 4, contributes to the reduction of a specific resistivity and hence the improvement of conductivity.
Metal or metal silicide formed on the gate electrode is however directly exposed to an atmosphere during semiconductor manufacturing processes so that it is oxidized to form an oxide layer 6 or it is etched to prevent a reduction of resistivity.
In particular, metal or metal silicide absorbs oxygen in the atmosphere and is oxidized during the manufacturing processes, thus forming an oxide layer on the surface thereof. FIG. 3 is a cross section showing a semiconductor device in which an interconnection layer made of a multilayer consisting of a polysilicon layer 4 and of a metal silicide layer 5 is formed with an oxide layer 6, and an aluminum interconnection 10 is formed at a contact hole after an insulation layer 9 is deposited upon the oxide layer 6. In this case, an ohmic contact is hindered due to the presence of the insulating oxide layer 6 at B between the aluminum interconnection 10 and the metal silicide layer 5.
As a result, the oxide layer at the contact hole has to be removed and is subjected to a reactive process with hydrofluoric acid after the reactive ion etching (RIE) process for opening the contact hole.
However, the metal oxide and metal silicide are prone to be etched with hydrofluoric acid. Thus, without a proper etching control, not only the metal oxide layer 6, but also the metal silicide 5 is etched as shown at A of FIG. 2 so that a reduction of resistivity cannot be achieved.
Furthermore, a conventional metal silicide layer as of a polycide construction involves a problem in that it has a large residual stress and a stepped portion thereof is apt to be cut.